1. Field of the Invention
The present invention relates to a display device and a method of fabricating a display device, and more particularly, to an organic electroluminescent display device and a method of fabricating an organic electroluminescent display device.
2. Discussion of the Related Art
In general, organic electroluminescent display (OELD) devices have an electron supply electrode, which is commonly referred to as a cathode, and a hole supply electrode, which is commonly referred to as an anode. The electrons and the holes are supplied to an electroluminescent layer from the cathode and anode, respectively, wherein each pair of the electrons and holes form an exciton. The OELD devices emit light when energy levels of the excitons are reduced from an excited state to a ground state. Accordingly, since OELD devices do not require additional light sources, such as a backlight device as in liquid crystal display (LCD) devices, both volume and weight of the OELD devices may be reduced. In addition, the OELD devices have low power consumption, high luminance, fast response time, and low weight. Presently, the OELD devices are commonly implemented in mobile telecommunication terminals, car navigation systems (CNSs), personal digital assistants (PDAs), camcorders, and palm computers. In addition, since manufacturing processes for the OELD devices are relatively simple as compared to LCD devices, manufacturing costs can be reduced.
The OELD devices may be classified into passive matrix-type OELD devices and active matrix-type OELD devices. Although the passive matrix-type OELD devices have simple structures and manufacturing processes are simple, they have high power consumption and are not suitable for large-sized display devices, and their aperture ratios decrease as a total number of electrical lines increase. On the other hand, the active matrix-type OELD devices have high light-emitting efficiency and high image display quality.
FIG. 1 is cross sectional view of an OELD device according to the related art. In FIG. 1, an OELD device 10 has a transparent first substrate 12, a thin film transistor array part 14, a first electrode 16, an organic electroluminescent layer 18, and a second electrode 20, wherein the thin film transistor array part 14 is formed on the transparent first substrate 12. In addition, a second substrate 28 has a moisture absorbent desiccant 22. The first electrode 16, the organic electroluminescent layer 18, and the second electrode 20 are formed over the thin film transistor array part 14. The electroluminescent layer 18 emits red (R), green (G), and blue (B) colored light, and it is commonly formed by patterning organic material within each pixel region “P” for the R, G, and B colored light.
The OELD 10 is completed by bonding the first and second substrates 12 and 28 together by disposing a sealant 26 between the first and second substrates 12 and 28. The moisture absorbent desiccant 22 on the second substrate 28 removes any moisture and oxygen that may have infiltrated into an interior of the OELD 10. The moisture absorbent desiccant 22 is formed by etching away a portion of the second substrate 28, filling the etched portion of the second substrate 28 with moisture absorbent desiccant material, and affixing the moisture absorbent desiccant material with a tape 25.
FIG. 2 is a plan view of a thin film transistor array part of an OELD device according to the related art. In FIG. 2, each of a plurality of pixel regions “P” defined on a substrate 12 includes a switching element “TS,” a driving element “TD,” and a storage capacitor “CST.” The switching element “TS” and the driving element “TD” may be formed by combinations of more than two thin film transistors (TFTs), and the substrate 12 is formed of a transparent material, such as glass and plastic. In addition, a gate line 32 is formed along a first direction, and a data line 34 is formed along a second direction perpendicular to the first direction, wherein the data line 34 perpendicularly crosses the gate line 32 with an insulating layer provided therebetween. A power line 35 is formed along the second direction, and is spaced apart from the data line 34. The TFT used for the switching element “TS” has a switching gate electrode 36, a switching active layer 40, a switching source electrode 46, and a switching drain electrode 50. The TFT used for the driving element “TD” has a driving gate electrode 38, a driving active layer 42, a driving source electrode 48, and a driving drain electrode 52. Accordingly, the switching gate electrode 36 is electrically connected to the gate line 32, and the switching source electrode 46 is electrically connected to the data line 34. The switching drain electrode 50 is electrically connected to the driving gate electrode 38 through a contact hole 54, and the driving source electrode 48 is electrically connected to the power line 35 through a contact hole 56. The driving drain electrode 52 is electrically connected to a first electrode 16 within the pixel region “P,” wherein the power line 35 and a first capacitor electrode 15, which is formed of polycrystalline silicon, form a storage capacitor “CST.”
FIG. 3 is a cross sectional view along III-III of FIG. 2 according to the related art. In FIG. 3, the OELD device has a driving thin film transistor (TFT) “TD” and an organic electroluminescent (EL) diode “DEL.” The driving TFT “TD” has a driving gate electrode 38, a driving active layer 42, a driving source electrode 56, and a driving drain electrode 52. In addition, a first electrode 16 is formed over the driving TFT “TD” and is connected to the driving drain electrode 52 with an insulating layer 57 therebetween. The organic EL diode “DEL” includes the first electrode 16, an organic electroluminescent (EL) layer 18, and a second electrode 20. The organic EL layer 18 is formed on the first electrode 16 for emitting light of a particular color wavelength, and the second electrode 20 is formed on the organic EL layer 18. A storage capacitor “CST” is connected in parallel to the driving TFT “TD,” and includes first and second capacitor electrodes 15 and 35. The driving source electrode 56 contacts the second capacitor electrode 35, i.e., a power line, and the first capacitor electrode 15 is formed of polycrystalline silicon material under the second capacitor electrode 35. The second electrode 20 is formed on the substrate 12 upon which the driving TFT “TD,” the storage capacitor “CST,” and the organic electroluminescent layer 18 are formed. Adjacent pixel regions may be divided by a sidewall.
OELD devices are classified into bottom emission-type OELD devices and top emission-type OELD devices according to a transparency of the first and second electrodes 16 and 20 of the organic EL diode “DEL.” While the bottom emission-type OELD devices have high image stability and variable fabrication processing due to encapsulation, they are not adequate for implementation in devices that require high image resolution due to limitations of increased aperture ratio. On the other hand, since top emission-type OELD devices emit light upward through the substrate, the light can be emitted without influencing the TFT array part that is positioned under the organic EL layer. Accordingly, design of the TFT may be simplified and aperture ratio can be increased, thereby increasing operational life span of the OELD device. However, since a cathode is commonly formed over the organic EL layer in the top emission-type OELD devices, material selection and light transmittance are limited and light transmission efficiency is lowered. If a thin film-type passivation layer is formed to prevent a reduction of the light transmittance, the thin film-type passivation layer may fail to prevent infiltration of exterior air into the device.